Polymer-bound catalysts hydrogenation

Scheme 4.31 Polymer-bound catalyst (51) in hydrogen transfer reactions.

Continuous homogeneous catalysis is achieved by membrane filtration, which separates the polymeric catalyst from low molecular weight solvent and products. Hydrogenation of 1-pentene with the soluble pofymer-attached Wilkinson catalyst affords n-pentane in quantitative yield A variety of other catalysts have been attached to functionalized polystyrenes Besides linear polystyrenes, poly(ethylene glycol)s, polyvinylpyrrolidinones and poly(vinyl chloride)s have been used for the liquid-phase catalysis. Instead of membrane filtration for separating the polymer-bound catalyst, selective precipitation has been found to be very effective. In all... 

Polymer bound catalyst systems have become highly sophisticated. For supports, commercial resins have in cases given way to custom tailored polymers designed to optimize a supported catalyst s performance. This progression from simple to complex systems is illustrated by advances in supported catalysts used for the asymmetric hydrogenation of enamides. 

The solution to this problem has been to attach these catalysts to polymer supports. The ideal polymer-bound catalyst must satisfy a formidable list of requirements. It should be easily prepared from low cost materials. The support must be compatible with the solvent system employed, and be chemically and thermally stable under the reaction conditions. The catalyst should show minimal losses in reaction rate or selectivity when bound to the support, and should be able to be recycled many times without loss of activity. Finally, the interactions between the catalytic site and the support must be either negligible or beneficial. The development of polymer supported rhodium-phosphine catalysts for the asymmetric hydrogenation of amino acid precursors illustrates the incremental process which has led to supports which approach the ideal support. 

All of the polymers swell in both tetrahydrofuran and ethanol. Since ethanol would be expected to compete with the polymer bound alcohols for sites at the catalyst, tetrahydrofuran was chosen as the reaction solvent. The polymer bound catalysts were prepared by stirring the polymer and /x-dichlorobis(l,5-cyclooctadiene) dirhodium (I) in tetrahydrofuran for several hours, and after filtration the yellow catalyst was then transferred under argon to the reaction vessel containing the substrate. Solvent was added, and the reaction vessel was pressurized with hydrogen. At the end of the reaction, the pressure was released and the product was iso-... 

Recently a rhodium water-soluble polymer-bound catalyst, based on the commercially available copolymer of maleic anhydride and methyl vinyl ether, was shown to be very active in the hydrogenation of various substrates in basic aqueous media [25]. 

The metal complexes most often studied as polymer-bound catalysts have been Rh(I) complexes, such as analogues of Wilkinson s complex. The catalytic activity of a bound metal complex is nearly the same as that of the soluble analogue. Rhodium complexes are active for alkene hydrogenation, alkene hydroformylation, and, in the presence of CH3I cocatalyst, methanol carbonylation, etc. Polymer supports thus allow the chemistry of homogeneous catalysis to take place with the benefits of an insoluble, easily separated catalyst . ... 

Last but not least, the success of aqueous-phase catalysis has drawn the interest of the homogeneous-catalysis community to other biphasic possibilities such as or-ganic/organic separations, fluorous phases, nonaqueous ionic liquids, supercritical solvents, amphiphilic compounds, or water-soluble, polymer-bound catalysts. As in the field of aqueous-phase catalysis, the first textbooks on these developments have been published, not to mention Job s book on Aqueous Organometallic Catalysis which followed three years after our own publication and which put the spotlight on Job s special merits as one of the pioneers in aqueous biphasic catalysis. Up to now, most of the alternatives mentioned are only in a state of intensive development (except for one industrial realization that of Swan/Chematur for hydrogenations in scC02 [2]) but other pilot plant adaptations and even technical operations may be expected in the near future. 

Linear polystyrene has also been used to support asymmetric hydrogenation catalysts containing chiral diphosphine rhodium(I) complexes (50). Asymmetric hydrogenations of itaconic acid were carried out, forming (R)-2-raethylbutanedioic acid with e.e. s ranging from 20-37%. None of the polymer-bound catalysts were more effective than (-)-DIOP-RhCl and the observed e.e. s were found to be dependent on the molecular weight of the polymer chain, its raicrostructure and solubility. 

Interest in the hydrogenation reaction is not, however, limited to a documentation of the organic transformations that are possible. This reaction has also been used more than any other to probe the characteristics of polymer-bound catalysts to determine the details of the effects of catalyst attachment. 

Since the researcher normally looks to the chemistry of soluble complexes in designing polymer-bound catalysts, it is notable that some areas that have proven fruitful in homogeneous catalysis have been omitted from investigations using polymer-bound catalysts. One of these areas concerns the reactions of arenes. Benzene, for example, may be hydrogenated with homogeneous cobalt phosphite and ruthenium phosphine complexes, but the corresponding supported versions are not reported. Aryl halides may be carboxylated in the presence of a soluble palladium catalyst ... 

Atactic poly(methallyl)alcohol reacts with PClPha to give a polyphosphite Pn, and on treatment with [Rh2(Cl)2(CO)4] or [RhaCCOaCnorbornadienela] polymer-bound catalysts are obtained which are active in the hydrogenation of a variety of olefins. The species derived from the norbornadiene complex catalyses oct-l-ene hydrogenation at a rate dependent on catalyst concentration with solutions 0.125 mol 1 in olefin and at a rate oc [olefin] / up to a concentration of 0.3 mol 1. No attempt is made to rationalize these results in mechanistic terms. 

Tris(triphenylphosphine)chlororhodium (I) (Wilkinson s catalyst) has been the most widely studied homogeneous hydrogenation catalyst. Analogous polymeric catalysts have been prepared for use in hydrogenation. A variety of alkenes, cycloalkenes, dienes, and alkynes have been reduced by polymer-bound catalysts, indicating the wide scope of their use (Grubbs et al., 1977). 

A polymer-bound hindered amine light stabilizer [P-HALS] has been synthesized by terminating the living anionic polymerization of isoprene with 4(2,3-epoxy pro-poxy)-1,2,2,6,6-pentamethylpiperidine followed by hydrogenation of the resulting polymer to E-P copolymer using Zeigler type catalyst [40] ... 

The polymers were converted to supported catalysts corresponding to homogeneous complexes of cobalt, rhodium and titanium. The cobalt catalyst exhibited no reactivity in a Fischer-Tropsch reaction, but was effective in promoting hydroformylation, as was a rhodium analog. A polymer bound titanocene catalyst maintained as much as a 40-fold activity over homogeneous titanocene in hydrogenations. The enhanced activity indicated better site isolation even without crosslinking. 

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